feat: Implement alert creation service with FCM notifications

- Add AlertCreationService to handle alert creation and notifications.
- Implement idempotency key support to prevent duplicate alerts.
- Create alert notification service for sending FCM messages.
- Add tests for alert creation and notification services.
- Document FCM notification flow and alert creation process.

backend/package.json

@@ -26,6 +26,7 @@
     "dotenv": "^16.3.1",
     "express": "^4.18.2",
     "express-rate-limit": "^7.1.5",
+    "firebase-admin": "^14.1.0",
     "helmet": "^7.1.0",
     "jsonwebtoken": "^9.0.3",
     "multer": "^2.0.2",